The idea that Peter and MJ married to have kids is nonsense. Peter and MJ aren't against having kids, but they were never rushing or dying to have kids either. The only exception was during the Clone Saga when they were written out. (It's also why I don't mind 616 retconning Baby May, since it wasn't a natural transition).
Also, "all the stories still happened only they weren't married" still doesn't explain why they broke up. If the only thing that didn't happen is them signing the marriage papers, how does that mean they broke up? Does legal status count more than the bond two people developed? Marriage is an important mileage, yes, but even most married couples wouldn't say that signing the papers was the end-all-be-all of their relationship.
OMD basically told the world that a couple can't function or be together indefinitely if they don't get married. And OMIT told the world that marriage is only for having kids. Both are 1950's preconceived notions, not at all compatible with Peter and MJ or even with most New Yorkers in their age group today.